Permanent magnet dynamoelectric machines, per se, are, of course, well known in the art, wherein relative rotation occurs between a stator and an armature. On such machines one of the stator and armature includes a plurality of permanent magnet poles, and the other has windings of electrically conductive wire. Normally, the stator has the permanent magnets mounted thereon, and the wound armature is rotated within the stator.
Between the armature and the stator poles, an air gap is maintained, whereby the magnetic field of the permanent magnets acts with a field generated by the supply of electricity to the electrically conductive armature windings to produce rotary motion in a DC motor, or with rotary motion supplied to the armature to induce current within the armature windings in a generator. In the motor, the current flow in the armature windings forces the relative motion between the stator and the armature, whereas in the generator, relative motion within the stationary field generates current.
When a dynamoelectric machine armature has current flowing through its conductive windings, a magnetic flux is built up around the conductors to produce a second magnetic field in the space between the stator and the armature. The effect of the resultant combined magnetic field is the distortion of the main field. This distortion is know n as armature reaction. Moreover, the armature reaction varies depending on the amount of armature current flowing in the conductors, known as the load, and the direction of current flow. The larger the current flow or load, the greater the armature reaction.
The armature reaction affects the permanent magnetic poles along their interior circumferential pole face by increasing the magnetic flux density toward one edge thereof while decreasing the magnetic flux density toward the other edge. The edges referred to are those encountered in the direction of rotation of the armature, wherein the first edge encountered is the leading edge and the second edge is the trailing edge. In a D.C. motor, the flux density of the leading edge is increased while the trailing edge flux is decreased. In a D.C. generator, the opposite is true. This decrease of magnetic flux at either the leading or trailing edge, if excessive, will result in a demagnetization of the permanent magnet material at that edge.
In order to prevent the demagnetization of the permanent magnet material resultant from armature reaction in dynamoelectric machines, it is well-known to utilize magnets that have a sufficient strength at the edges thereof so as not to be demagnetized by the armature reaction. Typically, techniques for designing the magnets to resist demagnetization include increasing the thickness of the magnet throughout in the radial direction, and/or reducing the circumferential length of the magnets. The total length of the flux path due to the armature reaction, including the magnet thickness and the air gap, must counter or use up the demagnetization force. However, these techniques disadvantageously result in increased machine size and weight, along with a decrease in machine efficiency.
It is also known, in a manner to resist demagnetization, to include additional elements such as shields or plates on or attached with the permanent magnets. See, for example, U.S. Pat. No. 4,471,252 dated Sept. 11, 1984 to West. Disclosed in West is a dynamoelectric machine with permanent magnets modified by the addition of shields provided within a recessed portion in the pole face of each permanent magnet while maintaining a constant air gap width between the rotor and the stator. The shield functions to redistribute the flux density in the part of the pole covered by the shield to resist demagnetization.
Another similar machine is disclosed in U.S. Pat. No. 4,639,625 to Abukawa et al., dated Jan. 27, 1987. The D.C. machine includes permanent magnets with high saturation magnetic flux density plates secured in recesses thereon, wherein the magnets and plates together define a substantially constant circumferential air gap between the poles and a rotor. The above Abukawa et al. patent and U.S. Pat. No. 4,554,474 dated Nov. 19, 1985 to Morishita et al further disclose the use of auxiliary poles on the leading edge side of a permanent magnet pole of a D.C. dynamoelectric machine.
A different attempt for providing a permanent magnet which resists demagnetization in a dynamoelectric machine is disclosed in U.S. Pat. No. 4,110,718 to Odor et al., wherein each permanent magnet pole comprises a composite magnet with the material at the demagnetization edge (i.e., the trailing edge) is made of a magnetic material having high coercive force.
These known devices disadvantageously require production by processes including additional steps in the preparation of recesses, the attachment of plates or shields, and the formation of composite magnets. Such processes also disadvantageously increase associated manufacturing costs without significant improvement in the reduction of dynamo size and weight.
With the introduction of high-strength permanent magnets, such as disclosed in U.S. Pat. Nos. 4,104,787 and 4,151,435 to Jandeska et al., dated Aug. 8, 1978 and Apr. 24, 1979, respectively, it has become increasingly possible to reduce dynamoelectric machine size. These smaller dynamos are particularly found to be useful in an automotive environment such as an automotive window controller. How ever, to compensate for demagnetization from armature reactance flux, it is still necessary that the magnets be thick enough to resist demagnetization force. The increased thickness of such magnetized material is extremely costly, even though the magnet is overall very thin. Likewise, U.S. Pat. No. 4,453,097 to Lordo, dated June 5, 1984, utilizes high-strength permanent magnets which rely on the high coercive force of the magnet material to resist the armature reaction field, without modifying the magnets. In other words, the magnets have a sufficient and constant thickness to resist demagnetization.
Clearly then, there is a need for a high-strength permanent magnet which sufficiently resists demagnetization flux from armature reaction at the edges thereof, provides a strong field for torque or current production, and minimizes the costs of production and material associated with high-strength permanent magnets.
Another effect of the armature reaction is the shifting of the neutral plane of the dynamoelectric machine when the machine is loaded. As before, loaded refers to the supply or generation of current in the conductive windings of the armature when used as a motor or generator respectively.
In a D.C. motor, direct current is supplied to the armature windings conventionally by brushes and a commutator. The purpose of the brushes and commutator is to switch current direction to the armature windings (as is well known), but they also necessarily short circuit the loop passing through the neutral plane. It is desirable that the brush short circuit that loop at the instant it lies within the neutral plane, so that during commutation there is a minimum interaction with the field flux in the loop and the potential difference across the loop is at a minimum. When the short circuiting is at the neutral plane, sparking between the brushes and the commutator is effectively reduced or eliminated, thereby greatly increasing brush and commutator life. However, the armature reaction, as noted above, shifts the neutral plane to a degree depending proportionally to the load applied to the armature windings. In a D.C. motor, the direction of the shift is opposite to the direction of rotation of the armature. But, to achieve perfect commutation, it is necessary that the axis of the brushes coincides with the axis of the neutral plane. Therefore, as the dynamoelectric machine is loaded, either the brush axis has to be moved to the new position of the neutral plane, or something must prevent the neutral plane from moving.
It is the well known practice in the art to employ interpoles in the commutating zone between the main field poles of a dynamoelectric machine. Such interpoles are comprised of magnetically permeable poles (same as the field frame) connected to or integral with the field frame in line with the neutral plane between the main poles, wherein the interpoles include windings for producing a magnetic field having a corrective magnetic flux to counteract the induced voltages in the armature coils to be commutated. Interpole windings are connected in series with the armature windings, so that the armature current causes the interpole windings to set up magnetic fields around the armature. Moreover, the interpole fields are set up to cancel and even slightly overpower the effect of armature reaction, and to increase or decrease proportionally to the armature reaction as armature current is increased or decreased. In other words, the wound interpoles are self-regulating.
Examples of dynamoelectric machines utilizing interpoles connected in series with armature windings are disclosed in Boesel U.S. Pat. Nos. 4,435,664 dated Mar. 6, 1984, and Mercier U.S. Pat. No. 4,516,046 dated May 7, 1985. The Boesel patent further discloses modifications to interpoles for improving sparkless commutating with less influence on the main poles by shaping interpole tips and by including auxiliary windings. The Mercier patent discloses an arrangement whereby high-strength rare earth magnets are used as main poles with auxiliary wound poles to create a flux added to or subtracted from the main flux depending on motor velocity.
However, these known interpoles are limited in applicability to dynamoelectric machines which are sufficiently large to accommodate interpole windings. Typically, interpoles are only used in larger higher horsepower D.C. motors, where high torque output is required without concern for space conservation.
When dealing with the newest high-strength permanent magnet material, it is a main concern to use as little material as possible while maintaining a high flux field to generate sufficient torque. Moreover, these high-strength permanent magnets allow the manufacture of much smaller motors which are applicable, for example, to automotive equipment operating within very small confined spaces. The attempt to then improve the efficiency of the motors by including wound interpoles would absolutely defeat the above-stated purpose, because the windings would require more space and would increase motor size. Such an attempt is shown by the Mercier patent.
Moreover, as stronger magnets are used for the main poles, and the dynamoelectric machine diameters are therefor reduced, the distance between the armature surface and the inside diameter of the field frame becomes so short that many lines of armature flux can travel across the distance in the commutating zone. The increase in flux in the commutating zone will offset the efficiency obtained by the thinner higher strength magnets.
Clearly then, a D.C. dynamoelectric machine that utilizes high-strength magnets as the main poles for space conservation and that has improved efficiency due to the effective countering of armature reaction is needed.